My hamster Bunky used to use it loads but for the last 3 weeks or so now I haven't seen him touch it. I even lifted him onto the wheel and he still isn't interested.
He loves his ball though.
Bunky did fall off his wheel recently, but he's used it since. Maybe your ham has had an accident with it in the night, and although there's no visible sign of injury, and he didn't actually hurt himself, he's scared to use it?